Logic gates are important elements in many electronic devices, particularly those involving complex digital circuits. Transistors are the basic building blocks for implementation of logic gates and most, if not all, logic gates can be formed by beginning with an inverter circuit. Thin-film transistors are becoming a common tool in implementing logic gates, particularly in some technologies requiring smaller size such as, for example, thin-film displays, including electrophoretic displays, and the like.
There are generally two types of thin-film transistors (TFT) which are based on the underlying thin-film technology: p-channel devices are usually made from polymer/organic TFTs, and n-channel devices are usually made from, for example, hydrogenated amorphous silicon (a-Si:H) and transition-metal oxide (TMO) TFTs, such as, for example, InGaZnO (IGZO) TFTs. Polycrystalline silicon (poly-Si) TFTs are an exception in that poly-Si can be used to implement ambipolar transistors. However, in ambipolar transistors, performance non-uniformity can be a drawback due to grain boundary defects. Other drawbacks of ambipolar circuits may include high off currents so the switching action does not turn off completely and poor uniformity across large areas that may degrade image quality for large screens.
For the particular example of thin-film displays or panels, TFTs are key components in pixel circuits, digital signal processing and also in on-panel driving circuitry. For example, besides pixel circuits for driving the display media, TFTs are also used in level shifters, shift registers, line drivers, and other circuits.
Since complementary (i.e. ambipolar) transistors are difficult to provide when using TFTs, the design of an inverter with unipolar TFTs may require multiple inputs, and/or control signals in order to provide the desired operation. In particular, in attempts to provide rail-to-rail operation and decrease power dissipation, various inverter designs have been using dual gate devices or hybrid devices and complex fabrication processes but at the expense of cost, additional power consumption and yield.
These limitations can prevent full-swing digital circuits from being implemented for applications in emerging thin-film technologies such as flexible organic or transparent TMO electronics. In addition, even if a larger swing in voltage can be obtained, known circuits often have large direct path current which leads to substantial waste of power and energy.
In the case of thin film display panels, it is desirable to have capabilities of implementing analog and digital driving circuits fabricated on-panel due to the increased number of address/data lines in flat panel display and sensor array applications with a limited number of pads or physical connections. Currently, flat panel display drivers tend to use CMOS or other similar, expensive technologies on a separate chip and extra contact pads, and connections are used to pass the required signals to the panel. This may be a problem for higher resolution displays due to lack of space and, for mobile applications, small size. Higher resolution means more pixels, which means smaller pixels, which means less space to make connections to external CMOS driving circuits and chips. In addition, the use of such external drivers can lead to extra power and energy consumption.
Generally speaking, thin-film transistor (TFT) technology is relatively inexpensive to manufacture, yet in typical implementations has only one type of transistor (n-type or p-type). As a consequence, circuit implementations tend to have reduced output swings, high direct path current and power dissipation issues.
As such, there is a need for improved implementations of unipolar transistor logic gates, including inverter circuits, that address at least some of the deficiencies of conventional unipolar logic gates and inverter circuits.